Hydraulically powered triplex pump and control system therefor

ABSTRACT

A hydraulically powered triplex pump having at least three pumping units, each operable in a cycle including suction, precompression and discharge phases, with the cycles being out of phase with one another, whereby simultaneous performance of these functions results in a substantially constant pressure and flow of both the pumped fluid and the power fluid. Separate power and cycle control circuits, which may employ different fluids, are provided. Control valve assemblies, each including two sleeve valves communicating with a common chamber, are operated by the control circuit fluid to condition power circuit flow for various phases of the pumping cycle. Precompression valves, a fill valve and a dump valve are operated by the control circuit to provide additional conditioning of power circuit flow to provide the precompression phase and to correct for variations in pumping unit stroke length. A hydraulically operated shut off valve is employed to selectively isolate the multiplex pump from a source of pressurized power fluid. The power end of the pumping units include power cylinders which may be fluid interconnected at their rod ends so that operations in each power cylinder affect operations in the other cylinders.

United States Patent Cole [ Nov. 12, 1974 1 HYDRAULICALLY POWERED TRIPLEX PUMP AND CONTROL SYSTEM THEREFOR [75] Inventor: Clinton W. Cole, Duncan, Okla.

[73] Assignee: Halliburton Company, Duncan,

Okla.

22 Filed: Oct. 16, 1973 21 1 Appl. No.: 406,967

[52] US. Cl 417/342, 417/346, 137/596, 137/625.68

[51] Int. Cl. F04b 17/00, F04b 35/04 [58] Field of Search 417/342, 346, 339, 900

[56] References Cited UNITED STATES PATENTS 3,650,638 3/1972 Cole 417/346 3,773,438 11/1973 Hall et a1. 417/346 3,778,193 12/1973 Reinert 417/342 Primary Examiner-John J. Vrablik Assistant Examiner-Richard E. Gluck Attorney, Agent, or Firm-John H. Tregoning [57] ABSTRACT A hydraulically powered triplex pump having at least CONTEOL FLU/D 455EMBL Y three pumping units, each operable in a cycle including suction, precompression and discharge phases,

- results in a substantially constant pressure and flow of both the pumped fluid and the power fluid. Separate power and cycle control circuits, which may employ different fluids, are provided. Control valve assemblies, each including two sleeve valves communicating with a common chamber, are operated by the control circuit fluid to condition power circuit flow for various phases of the pumping cycle. Precompression valves. a fill valve and a dump valve are operated by the control circuit to provide additional conditioning of power circuit flow to provide the precompression phase and to correct for variations in pumping unit stroke length. A hydraulically operated shut off valve is employed to selectively isolate the multiplex pump from a source of pressurized power fluid. The power end of the pumping units include power cylinders which may be fluid interconnected at their rod ends so that operations in each power cylinder affect operations in the other cylinders.

3 Claims, 20 Drawing Figures PATENTEDHHV 12 1974 $847511 sum 02 0F 15 TQQ of an; NE

SHEET 05 0F 15 PATENTEDNUY 12 I974 PATENTED um 12 1914 saw a? or 15 PATENIEUMBY 12 1974 SHEET 08 [1F 15 PATENTED HEY I 2 I974 SHEET 09 0f 15 Pmminuummm I 3847.511

- sum 100E 15 PATENTEUuuv 12 1974 saw. 11 W15 P I NIEuxnuzmm 3.847511 sum 12UF15 HJUH UHL PATENTED NOV 12 I974 SHEET 13M 15 mums PATENTEUNUV 12 I974 SHEET HYDRAULICALLY POWERED TRIPLEX PUMP AND CONTROL SYSTEM THEREFOR BACKGROUND OF THE INVENTION This invention relates to pumps of the multiplex type. More particularly, but not by way of limitation, this invention relates to a triplex pump of the type in which the pump fluid is precompressed prior to discharge.

In the oil industry it has become common in the past to utilize multiplex pumps designed to deliver pumped fluid at high pressures on the order of 15,000 psi or greater. It has been found that even the slight compressibility of relatively incompressible pumped liquid medium may result in pulsating discharge pressure conditions since a portion of the power intended to accomplish the discharge phase of each pumping cycle is inherently utilized to first compress the liquid medium before it is brought to discharge pressure.

This discharge pattern is particularly undesirable where both high pumping pressure levels and very high delivery volumes are involved. The resulting pulsations could, under such conditions, subject the discharge conduits to severe vibrational forces. Thus, the pumping unit would be subject to stress conditions which might cause failure.

A significant advance in hydraulically powered triplex pumps has been disclosed in US. Pat. No. 3,650,638, entitled l-Iydraulically Powered Pump Having a Precompression Function and issued to Clinton W. Cole on Mar. 21, 1972. This patent is assigned to the assignee of the present invention. I

SUMMARY OF THE INVENTION The present invention contemplates an improved hydraulically powered pump employing an improved control system whichconstantly monitors the operation of the pump during each phase of the pumping cycle to immediately correct any variations from optimum operating conditions.

An object of the invention is to provide an improved triplex pump having a precompression function.

Another object of the invention is to provide a hydraulically powered triplex pump which provides nonpulsating suction flow as well as discharge flow.

A further object of the invention is to provide a fluid operated triplex pump and control system therefor which provides constant monitoring of the functioning of the pump and correction of variations from optimum operating conditions during each phase of the pumping cycle.

A still further object of the invention is to provide improved fluid controlled valving for the operation and control of a hydraulically powered triplex pump.

Still another object of the invention is to provide an improved hydraulically powered triplex pump having high volume and pressure capabilities which is readily mobile and economical in construction and operation.

The preferred embodiment of the invention comprises a hydraulically powered triplex pump having at least three pumping units each operable in a cycle including suction, precompression and discharge phases, with the cycles of each unit being out of phase with one another. The fluid end of the pump terminates in a common discharge line which is in fluid communication with the discharge end of each pumping unit. Like- I pressure and flow of the pumped fluid occurs in the common suction line and in the common discharge line.

Each pumping unit is fluid operated bypower fluid acting on a piston rod assembly extending between the fluid end and a power cylinder assembly at the power end of that unit. Each power cylinder assembly is connected through a control valve assembly to a common flow line communicating with a source of pressurized power fluid and a second common flow line communicating with a power fluid reservoir. By simultaneous performance of the suction, precompression and discharge phases of the cycle, a substantially constant pressure flow of the power fluid to and from these common flow lines is provided.

The control valve assemblies each include two sleeve valves communicating with a common chamber, which in turn communicates with a power cylinder. When one sleeve valve of a given control valve assembly is in an open position and the other is closed, pressurized power fluid enters the associated power cylinder assembly to provide a discharge function in the fluid end of the associated fluid end cylinder assembly. When the sleeve valves are in a reversed position, a suction function is permitted resulting in discharge of the power fluid in the power cylinder assembly to the power fluid reservoir. During a phase of the cycle when both of these sleeve valves are in their closed positions, a precompression valve is opened, and power fluid is directed to the power cylinder through the precompression valve.

A separate control circuit is utilized to move the sleeve valves to their desired positions.

The rod ends of the power cylinders are fluid interconnected so that the functions in each pumping unit are performed in response to those performed in the other units. Also, a portion of the power circuit is interrelated with the control circuit to provide for self-v correction of the stroke lengths in the power cylinder assemblies in response to control signals provided by the control circuit which constantly monitors the operation of the three pumping units throughout each phase of the pumping cycle.

Other objects and advantages of the invention will be evident from the following detailed description when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevational view partially broken away of a triplex pump according to the present invention.

FIG. 2 is a top plan view of the pump illustrated in FIG. 1.

FIG. 3 is a front elevational view of the pump illustrated in FIG. 1.

FIG. 4 is a front elevational view of the pump similar to FIG. 3jwith portions removed to more clearly illustrate the control valve assemblies associated with the power end of the pump and the location of the precompression, fill and dump valves.

FIG. 5 is a cross-sectional view of one control valve assembly.

FIG. 6 is a cross-sectional view of one precompression valve.

FIG. 7 is a cross-sectional view of the fill valve.

FIG. 8 is a cross-sectional view of the dump valve.

FIG. 9 is a cross-sectional view of the shut off valve.

FIG. 9A is an enlarged partial cross-sectional view illustrating valve seat construction of the shut off valve of FIG. 9.

FIGS. lOAI, 10Bl and 10C] are schematic illustrations depicting the control valve conditions, the power circuit flow and the power cylinder assembly functions respectively associated with the first, second and third phases of the pumping cycle.

FIGS. 10D1 and 10El are schematic illustrations depicting the control valve conditions, the power circuit fluid flow and the power cylinder assembly functions respectively associated with first and second variations in the operating conditions illustrated in FIG. 10A1.

FIGS. 10A2, 10B2, l(lC2,l0D2 and 10E2 are schematic illustrations showing the positions of the control circuit conditioning valves, the resulting movement of the control valves and the control circuit fluid flow which accomplish the control valve movement and the actuation of the precompression, fill and dump valves in the phases of the pumping cycle respectively associated with FIGS. 10A1, 10B1, 10Cl, 10Dl and 10121.

DETAILED DESCRIPTION Referring now to FIGS. 1 and 2, the triplex pump of the present invention is generally designated by the reference character 20.

The pump includes a fluid end assembly 22 comprising three substantially identical cylinders 24. The internal passages 26 of each of the fluid end cylinders 24 are each in communication with a valved pump cylinder head 28, each of which is provided with a suction check valve assembly 30 and a discharge check valve assembly 32. The discharge check valve assemblies 32 communicate with a common discharge manifold 34, providing one-way fluid flow therethrough from the cylinder heads 28 into the manifold 34 and the suction valve assemblies 30 communicate with a common suction header 36 providing one-way fluid flow therethrough from the suction header 36 into the cylinder heads 28. Each of the discharge check valve assemblies referred to is continuously available to yieladably resist fluid flow into the discharge manifold 34. The yieldable resistance is operable during the precompression stage ofa pumping cycle as well as during the discharge stage as will be described in detail hereinafter.

Extending from the fluid end assembly 22, in a direction away from the pump cylinder heads 28, is a power end assembly 38. The power end assembly includes three substantially identical power cylinders 40 each having an internal passage 41. Each of these power cylinders 40 is in generally longitudinal alignment with one of the fluid end cylinders 24. A piston rod assembly 42 extends longitudinally into each power end cylinder 40 and the respective aligned fluid end cylinder 24. The ends of the piston rod assemblies 42 which extend into the internal passages 26 of the fluid end cylinders 24 are provided with capped plunger ends which function as pumping pistons 44. The pumping pistons 44 are operable to bring about suction and discharge action in a conventional manner, and precompression action in a manner to be more fully described hereinafter.

The opposite ends, or power pistons, 46 of the piston rod assemblies 42 are in sliding and sealed engagement with the walls of the internal passages 41 of the power cylinders 40. In a manner to be more fully described, power fluid acts on opposite faces 48 and 50 of the power pistons 46 to reciprocate the piston rod assemblies 42.

The power end assembly 38 and the fluid end assembly 22 are separated by a spacer frameassemhly 5] which permits the fluid end piston rods, or plungers, 52 and the power end piston rods 54 to be separate members thereby facilitating maintenance operations. The rods 52 and 54 are each hollow, cylindrical members sealingly received in the fluid end internal passages 26 and the power end internal passages 41, respectively, as indicated at 55 and 56. If desired, a floating annular rod seal may be employed so as to allow the rods to operate slightly eccentric to the power cylinder bores. thereby eliminating the necessity of extremely accurate alignment between the power cylinders and the fluid end cylinders.

Extending longitudinally of and internally of the members 52 and 54 are tie-rods 57 which are joined at the outer ends thereof to the pistons 44 and 46 and at the inner ends thereof with a cam actuator 58 to form,

together with the rods 52 and 54 and their associated pistons 44 and 46, the integral piston rod assembly 42. The permissible stroke length of each piston rod assembly 42 is such that each cam actuator 58 is movable between a back position adjacent to a respective power end cylinder 40 and a forward position adjacent to the respective fluid endcylinder 24.

In a manner hereinafter more fully described, each cam actuator 58 is operable, in connection with a respective cycling valve FP, to provide a signal that a particular piston rod assembly 42 has reached its forward position. These cycling valves FP are mounted on the pump 20 at that forward position on a transverse valve mounting bar, indicated at 59. The location of this mounting bar 59 is such that the lengths of the valve engaging surfaces of the cam actuators 58 cooperate with and periodically remain in engagement with the valves F P in a tripped condition for a time sufficient to permit the necessary circuit functions to take place.

In a similar manner, each cam actuator 58 is operable, in connection with a precompression and stroke control valve or back position valve BP, to provide a signal that its associated piston rod assembly 42 has reached its back position. The precompression and stroke control valves BP are also mounted on the pump 20 at that back position on a transverse valve mounting bar, indicated at 60.

At the end of the power cylinders 40 remote from the fluid end assembly 22, each power cylinder 40 is in continuous communication with one of three identical control valve assemblies CV. To facilitate description of the pump of the present invention, the three control valves will be hereinafter referred to as l-CV, 2-CV and 3-CV, respectively. Similarly, the hereinafter described identical portions of the CV valve assemblies will be differentiated by the prefixes l, 2 and 3, as will the associated power cylinder assemblies 40.

The function of the control valves CV is to direct power fluid to and from the power cylinders 40 in a manner such that the power cylinders each operate on a suction, precompression, discharge cycle, each power cylinder 40 being out of phase with the others.

In the discharge phase of the cycle in a given power cylinder 40, power fluid acts on the outer face 50 of the power piston 46 to transmit force through the piston rod assembly 42 so as to cause the fluid end pumping piston 44 to move to its forwardmost stroke position whereby fluid in the cylinder head 28 is expelled through the discharge valve assembly 32 into the common discharge manifold 34. Prior to the discharge phase of the cycle, this fluid has been precompressed by power fluid acting on the power piston face 50 after passing through a precompression valve PR mounted on the control valve assembly CV. This precompression flow of power fluid causes the power piston 46, through the piston rod assembly 42, to move relatively slowly forward by an increment sufficient to compress the fluid to be pumped in the fluid end cylinder 24 and thereby raise the pressure of the fluid to approach the discharge pressure.

Suction movement of each power piston 46 is caused by power fluid acting on the inner face 48 of the power piston 46. The internal passages 41 of the power cylinders 40 are fluid interconnected in a normally closed circuit by a suitable common conduit 61. Thus, fluid in two rod ends of the passages 41, which fluid is displaced during precompression and discharge movement of the associated power pistons 463s caused to flow through the common conduit 61 into the third passage 41 to act on the inner face 48 of the power piston 46 in that third passage 41.

In this manner, the suction, precompression and discharge functions are simultaneously and responsively performed, one function being performed it? each fluid end cylinder 24. Therefore, constant pressure flow continually exists between the fluid end Of, f pump and the common suction header 3 and the" common discharge manifold 34.

For purposes of accomplishing automatic stroke correction, as hereinafter more fully described, the common conduit 61 connecting rod ends of the power cylinders 40 is in fluid circuit with a conventional accumulator 62 and with a source of power fluid through a normally closed fill valve, FV. The conduit 61 and the rod ends of the power cylinders 40 are also in fluid circuit with a power fluid reservoir, remote from the pump 20, through a normally closed dump valve DV. Also a rod end relief valve 64 provides selective communication between the rod ends of the power cylinders 40 and the remote power fluid reservoir in the event the power fluid pressure at the rod ends of the power cylinders exceeds a predetermined pressure.

As will be further described, the control valve assemblies CV, the precompression valves PR, the fill valve FV and the dump valve DV which direct the power fluid, preferably water, are, in the illustrated embodiment, monitored by a separate control fluid circuit. The control fluid circuit preferably utilizes a different fluid such as oil, air or a combination thereof. It will, however, be apparent that controls other than a control fluid circuit (e.g., an electrical sensing arrangement) may be utilized to monitor the power circuit. In the illustrated embodiment it will be assumed that oil will be employed as the control fluid.

It will be appreciated that the elements of the control circuit, the control valve assemblies CV, the power end assembly 38, the fluid end assembly 22 and the spacer frame assembly 51, may all be mounted'on a suitable common frame. In the illustrated embodiment, a tandem wheel semi-trailer 66 provides the common frame for supporting these elements. Preferably the mounting on the semi-trailer 66 is designed to permit the fluid end cylinders 24, the spacer frame assembly 51, and the power end cylinders 40 to move longitudinally relative to the semi-trailer for a limited distance, during reciprocation of the piston rod assemblies 42, so as to relieve any stresses on those members. Provision for such movement is made by slidably supporting the power end cylinders 40 within apertures formed in trailer crossmembers 68 and 70 and by slidably supporting the forward ends of the fluid end cylinders 24 within apertures formed in trailer crossmember 72 formed on the semi-trailer 66. The rearward ends of the fluid end cylinders 24 are rigidly secured to trailer crossmember 74 as are the cylinder heads 28. The fluid end cylinders 24, spacer frame assembly 51 and power end cylinders 40 are suitably joined by longitudinal tie bars 76. The transverse valve mounting bars 59 and 60 are each preferably secured to two of the uppermost longitudinal tie bars 76.

A suitable control fluid assembly 78 is mounted on the semi-trailer 66 below the power end assembly 38 and the spacer frame assembly 51. The control fluid assembly 78 includes a pump assembly 80, control fluid reservoir assembly 82, relief valve 84 and accumulators 86 and 88.

THE CONTROL VALVE ASSEMBLY Referring now to FIGS. 4 and 5, the previously identified identical control valve assemblies l-CV, 2-CV and 3-CV will be described.

From the cross-sectional view of one control valve assembly illustrated in FIG. 5, it will be seen that each control valve assembly includes a housing 90 having a longitudinal bore 92 extending therethrough. An inlet bore 94 and an exhaust bore 95 are formed in the housing 90 and communicate between the longitudinal bore 92 and the upper surface 96 of the housing 90. Counterbores 97 and 98 are formed respectively in the bores 94 and 95 forming respective annular shoulders 100 and 102 therein. Annular inlet and exhaust chambers 104 and 106 are formed respectively in the counterbores 97 and 98. A precompression passageway 107 is formed in the housing 90 and communicates between the inlet bore 94 and the front side 108 of the housing 90. A power fluid pasaageway 109 communicates between the inlet'chamber 104 and the front side 108 of the housing 90. The annular inlet chamber 104 communicates with the opposite vertical sides of the of the housing 90 through a pair of lateral inlet ports 110 formed irrthe housing 90 and positioned on opposite sides of the chamber 104. Similarly, the annular exhaust chamber 106 communicates with the vertical sides of the housing 90 through a pair of lateral exhaust ports 112 formed in the housing 90 and positioned on opposite sides of the chamber 106.

A cylindrically shaped inlet seat mount 114 is positioned within the counterbore 97 abutting the annular shoulder 100. A substantially identical exhaust seat mount 116 is positioned within the counterbore 98 abutting the annular shoulder 102. A substantially cylindrically shaped slotted inlet spacer 118 is positioned within the counterbore 97 abutting the upper end face of the inlet seat mount 114. An annular inlet valve seat 120 is securely mounted between the inlet seat mount 114 and the slotted inlet spacer 118. A substantially cylindrical slotted exhaust spacer 122 is positioned within the counterbore 98 and abuts the upper surface of the exhaust seat mount 116. An annular exhaust valve seat 124 is secured between the slotted exhaust spacer 122 and the exhuast seat mount 116.

An inlet cylinder assembly 126 and a substantially identical exhaust cylinder assembly 128 are mounted with their respective lower end portions disposed in the respective counterbores 97 and 98 of the housing 90. The lower end face 130 of the inlet cylinder assembly 126 abuts the upper end face 132 of the slotted inlet spacer 118. The loewr end face 134 of the exhaust cylinder assembly 128 abuts the upper end face 136 of the slotted exhaust spacer 122.

Each of the identical inlet and exhaust cylinder assemblies 126 and 128 includes a lower bearing and seal member 138 extending into the respective counterbores 97 and 98, a cylinder 140 extending upwardly from the lower bearing and seal member 138, an upper bearing and seal member 142 extending upwardly from the cylinder 140 and a cover 144 extending upwardly from the upper bearing and seal member 142 and capping the respective inlet and exhaust cylinder assemblies 126 and 128. The elements of the inlet and exhaust assemblies 126 and 128 are secured together by means of suitable annular V retainer couplings 146.

The inlet cylinder assembly 126 and the exhaust cylinder assembly 128 are each rigidly secured to the housing 90 by means of a plurality of threaded studs 148 extending through apertures in the cover 144 and received in threaded holes in the housing 90 and by threaded nuts 150 secured to the upper ends of the studs 148. A cylindrically shaped inlet sleeve valve member I is slidably disposed within the inlet cylinder assembly 126. An identical cylindrically shaped exhaust sleeve valve member E is slidably disposed within the exhaust cylinder assembly 128. The upper and lower outer cylindrical surfaces 152 and 154 of the valve members I and E are sealingly engaged by the annular seals 156 and 158 carried respectively by the upper and lower bearing and seal members 142 and 138 of the inlet and exhaust cylinder assemblies 126 and 128. Approximately midway of each of the valve members 1 and E there is provided an external cylindrically shaped flange 160 whcih engages the internal wall in the respective cylinder 140 of the inlet and exhaust cylinder assemblies 126 and 128. These flanges 160 function as pistons and define, together with an upper and lower cylindrical surfaces 152 and 154 of the valve members I and E and the upper and lower bearing and seal members 142 and 138 and their annular seals 156 and 158 located at opposite ends of each of the cylinders 140, upper and lower control fluid chambers A and B.

It will be appreciated that the internal diameters of the upper and lower bearing and seal members 142 and 138 are substantially equal to the diameters of the upper and lower outer cylindrical surfaces 152 and 154 of the valve members I and E. It will be further appreciated that the length of each of the valve members 1 and E is greater than the distance between the annular seals 156 and 158 of the upper and lower bearing and seal members 142 and 138. Thus, regardless of the position of the valve members 1 and E, the control fluid chambers A and B are continually isolated from power fluid passing through and controlled by the control valve CV;

When the valve member I is in its down or lower position, the lower end face 162 thereof is positioned proximate to an annular shoulder 164 of the inlet seat mount 114, preferably just short of abutment therewith. The outer periphery 1660f the valve member 1 adjacent to the end face 162 sealingly engages the annular inlet valve seat thereby blocking communication between the annular inlet chamber 104 and the longitudinal bore 92 of the housing 90.

Similarly, when the exhaust valve member E is in its down or lower position the lower end face 162 thereof is positioned proximate to an annular shoulder 168 of the exhaust seat mount 116, preferably just short of abutment therewith. The outer periphery 166 of the valve member E adjacent to the lower end face 162 sealingly engages the annular exhaust valve seat 124, thereby blocking communication between the annular exhaust chamber 106 and the longitudinal bore 92 of the housing 90.

It will be seen that when the communication between the annular inlet chamber 104 and the longitudinal bore 92 of the housing 90 is blocked by the inlet valve member 1, communication between the power fluid source and the power cylinder 49 is blocked. Similarly, when communication between the annular exhaust chamber 106 and the longitudinal bore 92 of the housing 90 is blocked by the exhaust valve member E, communicated between the power cylinder 40 and the power fluid reservoir is blocked. When the inlet valve member I is in the upper or open position, communication is open between the longitudinal bore 92 of the housing 90 and the source of power fluid. Similarly, when the exhaust valve member E is in the upper or open position, communication is open between the longitudinal bore 92 of the housing 90. and the power fluid reservoir.

Movement of the valve members 1 and E to the upper open position and the lower closed position to control power fluid flow through the control valve CV, is accomplished through the use of a pressurized control fluid. This control fluid causes such valve member movement by entering the cylinders of the inlet and exhaust cylinder assemblies 126 and 128 through control ports extending laterally therethrough. These control ports are labeled I-B, E-B, l-A and E-A respectively, and are defined by (unnumbered) communicating passages of different diameters.

Pressurization of the upper control ports l-A and E-A causes control fluid to enter the upper control chambers A of the inlet cylinder assembly 126 and the exhaust cylinder assembly 128. The valve members 1 and E are thus caused to move to their lower or closed positions, as illustrated by the valve member E in FIG. 5. Conversely, pressurization of the lower control ports [-8 and E-B moves the valve members I and E to their upper or open positions, as indicated by the valve member l in FIG. 5, the actuation of control fluid in the lower chamber B upon the pistons 160.

The valve members 1 and E are hydraulically cushioned or decelerated near the end of either an upward or a downward stroke by the restriction offered by the smaller of the two passages comprising the control ports A and B. When a given valve member nears completion of a shift in either direction, the larger passages 

1. A fluid operated triplex pump comprising: three piston and cylinder pumping assemblies, each operable in a suction, precompression and discharge cycle Out of phase with the others; each of said piston and cylinder assemblies including a power end and a fluid end, and a rod end; power circuit means for cyclically operating each of said piston and cylinder pumping assemblies in said suction, precompression and discharge cycles; control fluid circuit means for conditioning said power circuit means to cycle said piston and cylinder pumping assemblies; said power circuit means comprising: means for connecting the triplex pump to a power fluid reservoir for providing a source of power fluid; means for connecting the triplex pump to a power fluid pump for delivering pressurized power fluid from said fluid reservoir; flow regulating means operatively connecting the pressurized fluid delivered by said power fluid pump to said plurality of piston and cylinder assemblies and to said reservoir said flow regulating means including: a plurality of power circuit valve means, each for selectively placing one of said piston and cylinder pumping assemblies in unrestricted fluid communication with said pressurized power fluid, in restricted fluid communication with a reduced flow of said pressurized power fluid, or in fluid communication with said fluid reservoir; each of said power circuit valve means comprising: a power fluid inlet valve body having a power fluid entry passage and a power fluid exit passage; a power fluid exhaust valve body having a power fluid entry passage and a power fluid exit passage; precompression valve means having a power fluid entry port, a power fluid output port having a restricted passage, a control fluid pilot port, and means normally blocking the entry port from the output port when contorl fluid pilot pressure exists in the pilot port; the power fluid entry passage of said power fluid inlet valve body and the entry port of said precompression valve means being in continuous communication with pressurized power fluid delivered by said power fluid pump; a common chamber in fluid communication with the power fluid exit passage of said power fluid inlet valve body, and in continuous fluid communication with the power end of a respective one of said piston and cylinder assemblies, with the output port of said precompression valve means, and with the power fluid inlet passage of said power fluid exhaust valve body; the power fluid exit passage of said power fluid exhaust valve body being in continuous communication with said power fluid reservoir; the pilot ports of each of said precompression valve means being in control fluid communication with said control circuit means; power fluid inlet valve means carried by said power fluid inlet valve body for providing selective fluid communication between the power fluid entry passage and the power fluid exit passage of said power fluid inlet valve body; and power fluid exhaust valve means carried by said power fluid exhaust valve body for providing selective fluid communication between the power fluid entry passage and the power fluid exit passage of said power fluid exhaust valve body.
 2. The fluid operated triplex pump as defined in claim 1 wherein said control fluid circuit means comprises: opening and closing actuator chambers associated with each of said power fluid inlet and exhaust valve means: control circuit pump means for selectively delivering opening actuator control fluid to said opening actuator chambers of said power fluid inlet and exhaust valve means so as to place said power fluid inlet and exhaust valve means in their selective communication positions; and control circuit reservoir means for draining control circuit fluid from selected ones of said opening actuator chambers upon movement of selected ones of said power fluid inlet and exhaust valve means to their non-communicating
 3. A triplex pump as defined in claim 2 wherein: control circuit fluid is supplied to said opening actuator chambers of said power fluid exhaust valve means in two stages. 